Efficient transfer of signal power to or from an antenna often requires that the transceiver circuit impedance be conjugate-matched to the antenna feed-point impedance. The feed-point impedances of mobile station antennas vary significantly over the frequencies and frequency bands assigned for cellular applications. The large frequency variation of antenna impedance prevents effective signal power transfer across all bands and results in less than desirable signal-to-noise ratio (SNR) in both the uplink and downlink cellular transmissions. Feed-point impedance characteristics of mobile station antennas also vary with the surrounding environments of the antennas. Objects in near proximity to an antenna (e.g., hand, head, table-top) significantly change frequency-dependent impedance profiles of mobile station antennas.
Radio frequency (RF) amplifiers for wireless equipment also suffer problems with impedance matching. A common issue in implementation of a multi-band or multi-mode power amplifier is in achieving optimum performance from the transmitter power amplifier over a broad frequency range or for multiple waveforms. The gain, linearity, and power-added efficiency (PAE) performance of a power amplifier (PA) is heavily dependent on the complex load impedance presented to the transistors within the PA. Based on the characteristics of the transistors, a specific complex load impedance or narrow range of load impedance values will provide the optimum PAE. Often the optimum output power is achieved at a specific complex load impedance that is at a different impedance value than that required to achieve optimum PAE.
In addition, the optimum linearity performance in terms of error vector magnitude (EVM), adjacent channel power ratio (ACPR), or two-tone intermodulation ratio (TTIR) is achieved at possibly different load impedance than that required to achieve optimum PAE or output power. Because the final power amplifier in a radio transmitter is a dominant factor in the overall power consumption, efficiency, and linearity of the transmitter, it is normally critical to transform the actual impedance of the load through an impedance matching network to present the ideal load impedance to the power amplifier transistors depending on which performance parameter must be optimized.
Passive matching networks provide adequate matching at one or more frequencies. However, passive matching networks are unable to provide antenna or power amplifier matching across multiple frequency bands. For antennas, passive matching is unable to adapt to impedance changes due to changes in the antenna environment or surroundings. For power amplifiers, passive matching is unable to adapt to impedance changes needed for operations at different performance parameters.
Therefore, there is a need in the art for an improved impedance matching circuitry. In particular, there is a need for a for a control circuit for a tunable matching network that is capable of managing impedance within a radio.